1. Field of the Invention
The present invention relates generally to bonding or cladding sheets of different metals or alloys to one another to form a multilayered composite for various end users such as cookware, electrical contacts and like articles.
2. Description of the Related Art
Heretofore, it has been common to form composites of dissimilar metals by roll bonding. This technique is successful with certain metals but not all. Problems develop in delamination, particularly when certain objectionable types of oxides are present on the bonding surface(s) of adjacent plates or sheets. The present invention is directed to overcoming this problem.
In order to achieve a proper bond between metals by conventional roll or pressure bonding, it is necessary to provide clean surface contact between the adjacent surfaces to permit interatomic sharing of electrons between the dissimilar metals to be joined. Certain metals, such as, for example, carbon steel, copper, brass, titanium, zinc and certain aluminum alloys (2000, 3000 and 6000 series) possess oxide surfaces when exposed to the atmosphere which are difficult to bond to, unless expensive measures are taken. The oxides formed on these above-mentioned metals are relatively ductile and, therefore, deform and stretch when roll bonding is attempted. Due to their ductile nature, these oxide layers do not permit the bonding metal to contact the underlying bare substrate metal in order to form the desired interatomic bond alluded to above.
Heretofore, in order to join one of the above-mentioned metals to a dissimilar metal, it is conventional practice to first clean the surface of the metal and then place the metal in a furnace having a reducing atmosphere of, for example, cracked ammonia or carbon monoxide, or the like. Exposure to this furnace atmosphere for a given time will cause the removal of surface oxides. The metal plate or sheet is then quickly moved to a rolling mill and the material is roll bonded to a dissimilar metal. This conventional technique thus requires expensive special controlled atmospheric furnaces and the attendant safety and environmental problems associated with potentially hazardous gases. In addition, it is well-known that the unwanted oxide surfaces can re-form rapidly as the material is transferred from the furnace to the rolling mill. This reoxidation occurs especially as the composite plate enters the roll bite area of the rolling mill. In this roll bite area, the metal in the entry portion of the roll bite is caused to extrude rearwardly, which causes the adjacent plates to continuously separate so as to allow atmospheric oxygen to rapidly gush into the thusly formed gap, causing reoxidation of the clean metal surface. When the oxide forms to an appreciable extent, the roll bond is weakened and delamination can occur. The prior art has recognized this problem and has proposed a solution which involves welding the perimeter of the plates prior to rolling, which prevents the influx of atmospheric oxygen into the gap formed upstream of the roll bite area. This welding operation, however, represents yet another added expense in capital equipment and labor cost to the finished product.
The present invention provides a superior bonded article and a method of bonding a wide variety of metals and metal alloys by applying to at least one or both mating interfaces a thin layer of pure aluminum, then applying heat and pressure to achieve a bond. The thin aluminum layer is preferably applied by an electrolytic plating process or by a physical vapor deposition (xe2x80x9cPVDxe2x80x9d) technique.
The bonding of dissimilar metals has traditionally been achieved by either applying gross reduction at room temperature, lighter reduction in conjunction with heat or explosive bonding. In all of these cases, preparation of the surfaces is critical. As discussed above, surfaces must be clean and, in the case of many prior art metal combinations, free of oxides. The present invention employs the use of heat and pressure for bonding but differs from existing techniques in that the interface of metals to be bonded has at least one mating surface plated with pure aluminum to act as a bonding agent. Bonding by rolling, for example, is done without the need for special atmosphere controlled furnaces.
There is no need to provide a reducing or otherwise protective atmosphere, as required in the prior art, because the pure aluminum coating covers the objectionable ductile oxide coating on the first metal sheet or plate and strongly adheres thereto. The pure aluminum coating layer also naturally forms an oxide skin on its exposed surface, but aluminum oxide is a relatively brittle, non-ductile oxide. When the aluminum oxide skin is forced into facing contact with the second metal sheet or plate during roll bonding, the aluminum oxide skin readily fractures and exposes the underlying pure aluminum surface to permit the desired bare metal contact and the interatomic sharing of electrons between the adjacent metals to occur.
Thus, pure aluminum is a beneficial bonding agent for bonding dissimilar metals wherein one or both of the metal sheets or plates to be joined contain a ductile oxide surface. Certain metals, such as, for example, stainless steel and pure or EC (electrical grade) aluminum contain brittle oxide surfaces and do not require an aluminum coating layer to be applied prior to pressure bonding.